This paper attempts to contribute to a fuller understanding of the nature of language learner agency by considering it as a complex dynamic system. The purpose of the study was to explore detailed situated data to examine to what extent it is feasible to view learner agency through the lens of complexity theory. Data were generated through a series of in-depth interviews and narratives over a two-year period with a single, female tertiary-level EFL learner. The data were then analysed in a grounded manner taking a complexity perspective. The findings illustrate how agency can be conceived of as a complex dynamic system composed of a number of constituent components; each of which is itself a dynamic complex system. In particular, motivation, affect and self-regulation emerge as the ‘controlling’ components of this learner’s agentic system. The paper ends by discussing the findings in respect to pedagogy and future research.